The subject invention relates to optical inspection equipment used to evaluate parameters of thin films on semiconductor wafers. The subject device includes a cleaning module for reducing contaminants on the surface of the wafer prior to measurement to improve the accuracy and repeatability of the optical measurements. In the preferred embodiments, the cleaning module includes either or both of a microwave radiation source and a radiant heating source.
For many years, devices have existed for evaluating parameters of a semiconductor wafer at various stages during fabrication. There is a strong need in the industry to evaluate the parameters of multiple-layer thin film stacks on wafers using non-contact optical metrology tools. In these devices, a probe beam of radiation is directed to reflect off the sample and changes in the reflected probe beam are monitored to evaluate the sample.
One class of prior measurement devices relied on optical interference effects created between the layers on the sample or the layer and the substrate. In these devices, changes in intensity of the reflected probe beam caused by these interference effects are monitored to evaluate the sample. In many applications, the probe beam is generated by a broad band light source and such equipment is generally known as spectrophotometers.
In another class of instruments, the change in polarization state of the reflected probe beam is monitored. These devices are known as ellipsometers.
As thin films and thin film stacks have become more numerous and complex, the industry has begun developing composite measurement tools that have multiple measurement modules within a single device. One such tool is offered by the assignee herein under the name Opti-Probe 5240. This device includes a number of measurement modules including a broad band spectrophotometer and a single wavelength, off-axis ellipsometer. The device also includes a broadband rotating compensator ellipsometer as well as a pair of simultaneous multiple angle of incidence measurement modules. The overall structure of this device is described in U.S. Pat. No. 5,798,837, issued Aug. 25, 1998. The Opti-Probe device is capable of measuring information about ultra-thin films and thin film stacks with a high degree of precision.
There is a trend in the semiconductor industry to utilize very thin layers. For example, today, gate dielectrics can have a thickness less than 20 xc3x85. It is anticipated that even thinner layers will be used. There is a need to measure the thickness of these very thin layers with a precision and repeatability to better than 0.1 xc3x85. While the Opti-Probe device is capable of making such measurements with the necessary precision, problems have arisen with respect to repeatability, especially with ultrathin films. Repeatability means that if the same measurement is made at two different times, the same result for layer thickness will be produced.
After considerable investigation, it has been determined that variations in measurements over time is strongly affected by atmospheric conditions such as temperature, humidity and exposure time to the air. For example, the measured layer thickness could be considerably higher when the humidity is relatively high. The variation in measurement due solely to atmospheric conditions can exceed 1 xc3x85 which substantially reduces the likelihood of making repeatable measurements with a precision of 0.1 xc3x85 or better.
It is believed that the variation in measurements is due to the growth of a layer of hydrocarbons on the surface of the wafer. It is believed that this growth is related to water molecules that attach to the wafer surface when the wafer is exposed to normal atmospheric conditions. Since water is a polar molecule, we believe it may attract hydrocarbon molecules such that the contamination layer is actually associated with water. In order to improve the repeatability of the measurements results, it would be desirable to remove the contaminant layer prior to measurement.
There are many types of wafer cleaning procedures used in a semiconductor fabrication facility. However, any cleaning procedures which requires contact with the wafer, such as cleaning solutions, would not be desirable at this stage of fabrication since it can damage or contaminate the gate dielectric or the wafer. Additionally, most chemical cleaning processes require a drying cycle during which time a new hydrocarbon contamination layer could reform. High temperature baking is a non-contact cleaning approach that could be used to drive off a contaminant layer. However, the temperature required to clean the wafer would be relatively high and this high temperature can damage the gate dielectric.
Accordingly, it is an object of the invention to provide a method of evaluating a semiconductor wafer which includes an initial cleaning step that does not have the drawbacks of the prior art approaches.
It is a further object of the subject invention to provide a new and improved optical metrology system which utilizes microwave excitation to clean contaminants off the wafer prior to or during measurement to improve the repeatability of the result.
It is still a further object of the subject invention to provide a new and improved optical metrology system which utilizes radiant heating to clean contaminants off the wafer prior to or during measurement to improve the repeatability of the result.
It is still another object of the subject invention to provide a cleaning system wherein microwave radiation and radiant heating can be combined to enhance the cleaning the effect.
It is still another object of the subject invention to provide a microwave or radiant heating cleaning system which can be combined with one or more additional cleaning modalities such as conductive heating, UV radiative cleaning or carbon dioxide pellet cleaning.
In accordance with these and other objects, the subject invention provides for a method and apparatus for analyzing the characteristics of a semiconductor wafer. In the invention, the wafer is subjected to microwave radiation or radiant heating for a time period sufficient to significantly and repeatably reduce the thickness of any layer of water related contamination which may be on the wafer.
When microwave energy is used, it may be pulsed to increase the localized heating while minimizing the overall heating of the wafer. The microwave treatment can be performed in air, or in an atmosphere of an inert gas such as nitrogen or in a vacuum.
Once the wafer has been treated by the microwave radiation or radiant heating, it can be measured using a conventional optical metrology tool. As described above, one or more probe beams are caused to reflect off the sample and changes in the reflected probe beam or beams are monitored to derive information about the sample. It may be desirable to perform the measurement in a vacuum or inert gas environment. It may also be desirable to make the measurement in the same chamber where the wafer was exposed to the cleaning step to minimize any build up of the contamination layer while the wafer is being transferred. It would also be possible to measure the wafer at the same time it is being exposed to the microwave radiation or radiant heating.
It is believed that the cleaning achieved using the microwave radiation is not purely a thermal effect. Rather, the microwave radiation excites the rotational bands of the water molecules helping to drive it into the vapor state thereby carrying off any associated hydrocarbon contamination.
If a radiant heat source is used, it should operate in the one to ten micron wavelength regime. It is believed that the cleaning effect achieved using a radiant heat source at this wavelength excites the vibrational bands in the water molecules helping to drive it into the vapor state thereby carrying off any associated hydrocarbon contamination.
In one embodiment of the invention, cleaning is achieved by combining the microwave radiation and the radiant heating. Using two cleaning sources permits the optimization of the parameters of both modalities to maximize cleaning while minimizing damage to the wafer or deposited thin films.
As discussed in more detail below, in certain situations, it may be beneficial to enhance the microwave or radiant heat cleaning of the wafer with other cleaning modalities. For example, supplemental heating of the wafer may be performed using a conductive or convection heat source. Certain other contaminants might be removed by bombardment of the wafer with UV radiation or a stream of frozen carbon dioxide pellets. Any of these cleaning modalities could be used alone or in combination with microwave or radiant heating to clean the wafer prior to measurement.
It may also be desirable to initially expose the wafer to a high humidity environment before cleaning the wafer with microwave energy. Such a high humidity environment would increase the amount of water molecules attached to the wafer. When the wafer is subsequently exposed to the microwave radiation, the vaporization of these added water molecules may actually enhance the removal of other contaminants thereby improving the cleaning action.
Further objects and advantages of the subject invention will become apparent from the following detailed description, taken in conjunction with the drawings, in which: